The present disclosure relates generally to systems and methods for magnetic resonance imaging (“MRI”). More particularly, the disclosure relates to systems and methods for separating signal contributions from two or more chemical species using MRI.
MRI uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field, such as the so-called main magnetic field, B0, of an MRI system, the individual magnetic moments of the nuclei in the tissue attempt to align with this B0 field, but precess about it in random order at their characteristic Larmor frequency, ω. If the substance, or tissue, is subjected to a so-called excitation electromagnetic field, B1, that is in the plane transverse to the B0 field and that has a frequency near the Larmor frequency, the net aligned magnetic moment, referred to as longitudinal magnetization, may be rotated, or “tipped,” into the transverse plane to produce a net transverse magnetic moment, referred to as transverse magnetization. A signal is emitted by the excited nuclei or “spins,” after the excitation field, B1, is terminated, and this signal may be received and processed to form an image.
Chemical-shift imaging (CSI) is a general approach for separation of multiple spin species in the MRI signal, which explores the differences in spin precession induced by the spin species' chemical shift with respect to the main resonance frequency. CSI methods collect images at multiple echo times, such that the phase shifts between the species can be used to evaluate parametric maps corresponding to each of the species.
One prominent CSI technique is Dixon separation of fat and water (F/W) signals, which explores chemical shift differences between fat and water for either creation of fat-free images or quantification of water and fat contributions to MRI signal. More complex variants of F/W imaging, such as iterative decomposition of water and fat with echo asymmetry and least-squares (IDEAL) model F/W contributions jointly with the magnetic field inhomogeneity through a field map and, for gradient echo sequences, T2* decay.
In these methods, several images are acquired with different echo time (“TE”) shifts, typically using a multi-echo spoiled gradient echo (“SPGR”) pulse sequence. Subsequently, separated water and fat images are reconstructed, and fat fraction maps are obtained. In order for the resulting fat fraction maps to accurately measure proton density fat fraction, multiple confounding factors in the acquired echo signals need to be addressed. These confounding factors include B0 inhomogeneities, T1 bias, noise bias, T2 decay, spectral complexity of the fat signal, and phase errors, such as those due to eddy currents.
Thus, the presence of confounding factors may impact the robustness and reproducibility of CSI-based techniques. Of particular concern, CSI, in general, and Dixon F/W separation methods, in particular, must contend with inherent uncertainties arising from the competing sources of off-resonance (i.e., fat chemical shift and field inhomogeneity). The associated local minima of the nonlinear objective function, when trapped therein, may lead to significant errors (e.g., swapped fat and water). Several approaches have been proposed to reduce these errors. The majority of algorithms exploit assumptions of field map smoothness, which may fail in cases of significant field inhomogeneities (like in near-metal imaging, tissue-air interfaces, etc.) or when discontinuity between different tissue areas is present (like in imaging of breast and thighs).
It would therefore be desirable to provide a method for chemical species signal separation that is more reliable and reproducible because it can better control the impact of off-resonance effects on the resulting images and/or quantifications.